A general problem of driven machines, such as conveyor apparatuses, for example pumps, compressors and fans, or such as pulverizers, crushers, motor vehicles, etc., is efficient, variable-speed operation or starting up under load, or operation of, for example, energy extraction installations up to a speed equal to zero. Furthermore, electrical machines are used as an example of drive machines or generators, but the principle applies to all possible types of drive machines, as well as to, for example, internal combustion engines.
The electrical drives and generators that are most frequently used at present are three-phase machines, such as, for example, asynchronous machines and synchronous machines that are operated essentially only with constant speed. Moreover, a three-phase machine and a network downstream of it must be designed to be correspondingly large so that they can deliver a desired driving torque from a standstill. Therefore, electrical machines for this reason will also instead be connected directly to a power system, often made in combination with a frequency converter as a variable-speed drive. Thus, variable-speed operation from speed zero can be implemented without heavily loading the power system, but the approach is expensive and linked to major efficiency losses. One more economical alternative that is better in comparison and also better in efficiency is the use of differential systems—for example according to AT 507 394 A. The basic limitation here is, however, that depending on the transmission ratio of the differential stage, only a relatively smaller speed range can be achieved, and therefore in the so-called differential mode, i.e., in the case of the speed changes using the differential drive at the operating speed of the drive machine, essentially lower speeds on the drive shaft of a driven machine cannot be achieved.
There are several possibilities for doing this. According to DE 20 2012 101 708 U, for example, the transmission ratio of the differential gear can be fixed at 1. On this basis, the complete power train can be driven with the differential drive or the drive machine can be brought to synchronous speed and can be subsequently synchronized with the power system.
The disadvantage of this approach is that the differential drive and the frequency converter downstream of it are dimensioned to be much smaller than the drive machine and therefore can also only deliver a correspondingly small torque. This is not enough to accelerate the drive machine to the synchronous speed when the driven machine is in operation.
AT 514 396 A shows an approach with which drive machines can be accelerated into a speed range with high torque and the driven machine can be started up in a further step away from speed zero. This is done by the drive machine being started up from a speed of zero or roughly zero while an external braking torque acts on the drive shaft, and by the second drive being braked in an acceleration phase of the drive shaft. The disadvantage of this approach is that the braking device that is necessary for this purpose is complex and with a differential drive in a size on the order of, for example, 20% of the total system output, only a continuous speed range of roughly 50% to 100% of the working speed can be implemented.